Voltage-gated potassium (Kv), sodium (Nav), and calcium (Cav) channels regulate the flow of ions into and out of cells and thereby support specialized higher order cell functions such as excitability, contraction, secretion, and synaptic transmission (1). Hundreds of Kv, Nav and Cav, channel proteins provide the tremendous functional diversity required for the complex behaviors of eukaryotic vertebrate and invertebrate cell-types (2, 3). Ion channels are also widespread in prokaryotes but their gating and function are poorly understood because few have been functionally expressed in a system in which their properties can be studied.
Defects in the ion channels are responsible for a myriad of diseases including Long QT syndrome, which claims lives of about 4,000 children and young adults yearly in the United States alone. Ion channel blockers have been used to treat a number of cardiovascular diseases such as high blood pressure, angina, and atrial fibrillation. Atrial fibrillation alone affects over 2.5 million people in the United States, with an estimated 160,000 new cases diagnosed each year, and an estimated annual cost of $1 billion. Thus, ligands or drugs that modulate ion channel activity may be useful in treating these diseases.
The primary structural theme of ion-selective channels is of a pore region surrounded by two transmembrane (2TM) segments. The first high resolution images of a bacterial 2TM tetrameric channel revealed the structural basis of K+ ion selectivity encoded by the signature GY/FG sequence (4). In the primary structure of voltage-sensitive ion channels, an additional four transmembrane segments precede the pore-containing domain. The pore-forming subunits (α1) of Nav and Cav are composed of 4 homologous repeats of 6TM domains (3, 5). In theory, gene duplication of the 6TM K, or TRP channels provided the precise structural requirements for highly selective Na+ and Ca2+ channels. In particular, selectivity for Ca2+ requires coordination of the Ca2+ ions by four negatively charged glutamatic or aspartic acid residues lining the pore. The TRP class of ion channels are presumably tetramers of single 6TM subunits, but only a subset of these channels are moderately Ca2+ selective (6).
Eukaryotic ion channels, due to their complexity, have not been successful candidates for screening for compounds that modulate their activity. Therefore, there exists a need for ion channels that can be used in screening for compounds that modulate their activity and therefore used in the search for ligands or drugs to treat various disorders and defects, including central nervous system disorders, gastrointestinal disorders, and cardiovascular disorders.